1. Field of the Invention
The present invention relates to a high pressure isostatic pressing device and a method of providing a change of pressure between two pressure states in a high pressure isostatic pressing device in which a liquid pressure medium is used for generating the pressure. In particular, the invention concerns generation of pressure of at least 4000 bar.
2. Description of the Related Art
High pressure presses, in which a liquid pressure medium is used for generating isostatic high pressures, may be utilized for pressing different types of loads. For instance, high pressure presses are used for compacting powdered graphite or ceramic, or for treating foodstuffs. In certain applications, such as in treatment of foodstuffs, it is desirable to generate a pressure of about 6000 bar in the pressure vessel of the high pressure press in order to ensure the inactivating effect on micro-organisms and enzymes.
Standard hydraulic pumps of today can only be practically utilized to generate a pressure of about 4000 bar. The reason for this is inter alia dimensional limitations, such as fatigue limit of the materials of the pumps. Typically, steel is a construction material in these types of pumps and is subjected to considerable fatigue at the desirable high pressure level. Furthermore, the mechanical stress on certain components, such as valves and piping intersection points, would be very large at said high pressure level. If such a pump would be forced to generate a pressure of 6000 bar one or more components would soon break, already after a low number of pressure cycles.
One possible way to avoid the drawbacks mentioned above would be to use a single-stroke pressure intensifier, which has the advantage of having fewer components and being subjected to few press cycles. This means that its working life is considerably longer than the above mentioned standard pumps and thus achieves a better reliability. However, the disadvantage is that, for a single-stroke pressure intensifier to generate 6000 bar, it will be a large, bulky and expensive construction. Therefore, such a single-stroke pressure intensifier is most unpractical and is seldom used.
Another problem arises when the pressure vessel is to be decompressed after a finished pressing operation, i.e. when the liquid pressure medium is to be removed so as to obtain access to the pressed load inside the pressure vessel. To decompress the pressure vessel from 6000 bar to ambient pressure involves great wear and some fatigue on the release or decompression valve as the liquid pressure medium rushes at high speed past the valve.